You could also use DBeaver which provides exactly the function you're looking for (sorry, most of it is in German but I think it's self-explanatory; the tool itself also supports English):
To get this overview DBeaver fires this SQL to the database which you can of course execute manually and is quite similar to Malte's queries:
select r.*,p.ADMIN_OPTION,p.GRANTEE from EXA_DBA_ROLES r INNER JOIN EXA_DBA_ROLE_PRIVS p ON p.GRANTED_ROLE = r.ROLE_NAME ORDER BY P.GRANTEE,R.ROLE_NAME
Thanks for your quick reply!
I'd like to see the user in the first column and the corresponding role in the second column.
For instance, Peter Berg - DBA or Daniel Kahnemann - Developer.
Basically, this would be a join of the tables from above. However, I wonder how to do it.